In a typical inkjet recording or printing system, ink droplets are ejected from a nozzle at high speed towards a recording element or medium to produce an image on the medium. The ink droplets, or recording liquid, generally comprise a recording agent, such as a dye or pigment, and a large amount of solvent. The solvent, or carrier liquid, typically is made up of an aqueous mixture, for example, comprising water and one or more organic materials such as a monohydric alcohol, a polyhydric alcohol, or the like.
An inkjet recording element typically comprises a support having on at least one surface thereof at least one ink-receiving layer. There are generally two types of ink-receiving layers (IRL's). The first type of IRL comprises a non-porous coating of a polymer with a high capacity for swelling, which non-porous coating absorbs ink by molecular diffusion. Cationic or anionic substances may be added to the coating to serve as a dye fixing agent or mordant for a cationic or anionic dye. Typically, the support is a smooth resin-coated paper and the coating is optically transparent and very smooth, leading to a very high gloss “photo-grade” inkjet recording element. However, this type of IRL usually tends to absorb the ink slowly and, consequently, the imaged receiver or print is not instantaneously dry to the touch.
The second type of ink-receiving layer or IRL comprises a porous coating of inorganic, polymeric, or organic-inorganic composite particles, a polymeric binder, and optional additives such as dye-fixing agents or mordants. These particles can vary in chemical composition, size, shape, and intra-particle porosity. In this case, the printing liquid is absorbed into the open interconnected pores of the IRL, substantially by capillary action, to obtain a print that is instantaneously dry to the touch. Typically the total interconnected inter-particle pore volume of porous media, which may include one or more layers, is more than sufficient to hold all the applied ink forming the image.
Basically, organic and/or inorganic particles in a porous layer form pores by the spacing between the particles. The binder is used to hold the particles together. However, to maintain a high pore volume, it is desirable that the amount of binder is limited. Too much binder would start to fill the pores between the particles or beads, which would reduce ink absorption. On the other hand, too little binder may reduce the integrity of the coating, thereby causing cracking. Once cracking starts in an inkjet coating, typically at the bottom of the layer, it tends to migrate throughout the layer.
A porous inkjet recording medium that is glossy usually contains at least two layers in addition to the support: a base layer nearer to the support and a glossy image-receiving layer further from the support. One method of obtaining a “photographic-grade” gloss is to coat the inkjet receiving layers on a resin-coated paper support. Resin-coated paper support is relatively costly, however, and requires an extra resin-coating step in its manufacture.
For example, Bermel et al., U.S. Pat. No. 6,630,212, describes an inkjet recording medium comprising two porous layers coated on a resin-coated support paper. The two layers are coated simultaneously by a pre-metering method, extrusion hopper coating, on a polyethylene resin-coated support paper. The base-layer coating composition comprises fumed alumina particles, PVA binder, and coating aids at a solids content of 30%. The coated weight of the base layer is 43 g/m2. An image-receiving layer over the base layer comprises fumed alumina particles, cationic polymeric latex dispersion, and PVA binder. The coated weight of the IRL is 2.2 g/m2.
Inkjet, recording media with “photographic-grade” gloss can also be made when coating on a plain paper support. Because plain paper supports are generally rougher or less smooth than resin-coated paper supports, however, it is typically necessary to use special coating processes, such as cast coating or film transfer coating in order to achieve a smooth, glossy surface on the image receiving layer. These specialized coating methods are constrained in their productivity by drying considerations or by extra steps. Mild calendering with heat and pressure has also been used in combination with conventional blade, rod, or air-knife coating processes on plain paper in order to produce a glossy surface on the image-receiving layer, but these approaches tend to result in lower levels of gloss and smoothness than usually obtained for coatings on resin coated paper supports.
For example, a porous two-layer inkjet receiving material coated on plain paper support is described by Sadasivan et al., in U.S. Pat. No. 6,689,430. The inkjet recording element comprises a base layer coated from a composition at a solids level of 35% to form a layer with a dry weight of 27 g/m2. The base layer comprises inorganic pigments, namely precipitated calcium carbonate (PCC) and silica gel, and binders, namely polyvinyl alcohol and styrene-butadiene latex. One of the main functions of the base layer is to provide a sump for the ink fluids in the applied ink as compared to the colorant, whether dye or pigment-based. The image-receiving layer is coated over the dried base layer in the amount of 8.6 g/m2 using a coating composition of 15% solids comprising a mixture of colloidal alumina and fumed alumina particles, PVA binder, cationic polymeric latex dispersion, and coating aids. The inkjet recording element disclosed by Sadasivan et al., while providing good image quality and adequate gloss at moderate ink fluxes, is inadequate for higher printing speeds now demanded and is not as glossy as desired.
As the quality and density of inkjet images increases, so does the amount of ink applied to the inkjet recording element (also referred to as the “receiver”). For this reason, it is important provide sufficient void capacity in the medium to prevent puddling or coalescence and inter-color bleed. At the same time, print speeds are increasing in order to provide convenience to the user. Thus, not only is sufficient capacity required to accommodate the increased amount of ink, but in addition, the medium must be able to handle increasingly greater ink flux in terms of ink volume/unit area/unit time.
Porous glossy inkjet receiver materials that are commercially available at present generally comprise less than 50 g/m2 of porous ink-absorbing (or “ink-retaining”) layers and there is a limit to the ink fluxes that they can handle without a loss in image quality. The cost of high weight coatings using the materials, comprising fumed alumina, employed in the above-described example of U.S. Pat. No. 6,630,212 to Bermel et al. would be prohibitive in amounts beyond 50 g/m2. In addition, coating compositions comprising such materials thicken at high concentrations. On the other hand, coating of dilute compositions to achieve high weight coatings would require long driers, slower coating rates or multiple coating passes, all of which increase costs of facilities, energy, and/or labor and reduce productivity. Thus, the amount of ink absorbing material used in inkjet recording elements is currently limited as a matter of practice, in that the advantages of higher overall capacity of the coatings is outweighed by certain manufacturing problems and costs. In addition, it has not been demonstrated that high gloss can be obtained in porous inkjet recording elements without relatively expensive materials, or complicated or disadvantageous manufacturing processes. For example, inkjet media having base layers comprising calcium carbonate do not provide gloss and uniformity comparable with that of layers comprising mainly metallic oxide particles. Even with more expensive materials such as boehmite in the base layer, resin coated paper has been needed for high gloss.
In view of the above, the manufacture of high quality, high capacity, high gloss porous inkjet receiver materials has been complicated by multilayer structures, high coated weights of one or more layers, and relatively expensive materials or complicated processes.
Cuch, in US patent application publication 2004/0152819, describes a coating composition for preparing the undercoat of a glossy inkjet receiving material, comprising a mixture of 0 to 20% silica pigment and 80 to 100% fumed metallic oxide pigment. The receiver material may further comprise an optional overcoat comprising a mixture of 20 to 99% silica pigment and 1 to 80% fumed metallic oxide pigment, wherein the ratio of fumed metallic oxide to silica particles ranges from 1:200 to 4:1 Cuch teaches that the fraction of pigment comprising fumed metallic oxide should be greater in the undercoat than in the overcoat in order to obtain higher gloss. The layers are coated on either a paper support or a resin-coated paper which may have a smoothing layer prepared with a silica/calcium-carbonate pigment composition. The overall laydowns used by Cuch in the examples were less than 50 g/m2. The gloss levels depended on the base paper used among other factors, but unless specially calendered paper of high smoothness was used, the gloss at 60° was typically less than 50.